1. Field of the Invention
The present invention relates to the field of photolithographic mask making, and more particularly, to the fabrication of customized masks.
2. Background Information
Printed circuit boards are fabricated by providing an insulating substrate having a continuous layer of a metal such as copper foil bonded to one surface thereof and disposing a layer of photoresist over the metal foil and exposing the photoresist to light in the pattern which corresponds to the desired conductor pattern on the printed circuit board. This may result in cross-linking of the photoresist where it is exposed to light. This cross-linking reduces the solubility of the photoresist in appropriate solvents and the printed circuit board with the exposed resist is developed by immersion in a solvent for the photoresist material which differentially dissolves the unexposed and exposed portions of the photoresist. Following development of the photoresist, the printed circuit board is immersed in an etchant for the metallic foil of the printed circuit board which does not rapidly attack the photoresist with the result that the photoresist protects the metallic pattern where the photoresist remains and the remainder of the metal foil is etched away.
The exposure pattern applied to the photoresist is controlled by a photolithographic mask. Early in the photolithography art, such masks were hand fabricated by laying out dark strips of material such as plastic on a large matrix board (often 3 feet square or larger). The lengths and widths of these strips were in appropriate portion to the desired pattern on the printed circuit board. After the entire pattern was laid out, it was photographed and photographically reduced to the desired size for exposure of the photoresist on the printed circuit board blank. In this way, conductor patterns could be defined with sufficient accuracy and in a manageable manner. Each mask was time consuming and expensive to lay out. However, when amortized over many circuit boards, it was less expensive than hand wiring.
Early in the history of the semiconductor industry, a need developed for an ability to pattern layers on a semiconductor substrate in an accurate, detailed, repeatable manner with much finer resolution than is needed with printed circuit boards. Photoresist proved to be a solution to this problem. A layer of photoresist is disposed on the semiconductor wafer an exposed to a pattern of light which matches the desired pattern on the wafer either as a positive or negative, depending on whether a positive or negative photoresist is employed. The photoresist, upon exposure to light, changes its characteristics. With a negative photoresist, exposure to light results in cross-linking of molecules within the photoresist. In the case of a positive photoresist, the exposure to light breaks bonds within molecules of the photoresist. In either event, the photoresist layer is rendered differentially soluble in appropriate solvents. Following exposure, the photoresist is developed by immersion in an appropriate solvent which dissolves the portion of the photoresist layer which is not cross-linked without dissolving the portion of the photoresist layer which is cross-linked. The resulting pattern of photoresist matches the exposure pattern and is used to control the etching or other process steps to be carried out on the semiconductor wafer.
As the semiconductor industry matured, finer and finer patterns were needed with the result that mask production techniques became more refined, with step and repeat and laser drawing systems being developed to define a desired pattern in photoresist in a reasonable period of time with fine definition. Each mask is still expensive to design and fabricate. However, the mask which is used to expose the photoresist is used many times as part of the process of fabricating many identical devices. Thus, the cost of the mask is amortized over many devices which makes the cost of the mask acceptable.
As explained in the above-identified related incorporated-by-reference application, an area patterning mask can be used to control laser sculpturing of a corrective lens, other optical member for correcting vision or any other desired body. However, since the vision of each individual eye is essentially unique, when considered on a very detailed level, each mask can only be used once. Consequently, prior art mask fabrication techniques are unduly expensive and a less expensive, fast, accurate method of fabricating such masks is needed.
Liquid crystal displays have long been used to convert electronic information into human readable form. Early liquid crystal displays were employed as the display mechanism for electronic watches and similar large-character alpha-numeric displays. In such displays, rather coarse definition of the individual elements of the display was satisfactory because of the large size and low detail of the alpha-numeric characters.
As time has progressed, the resolution possible with liquid crystal displays has continually increased. Today, small television employ liquid crystal displays as their "picture tube". These TVs display a standard television picture and can change the entire image display in a short time which is generally less than 1/30th of a second. Thus, the liquid crystal display is a rapid transducer for converting electronic data into a pattern of light.
Liquid crystal displays are routinely available having cell sizes of about 100 microns.times.100 microns. Liquid crystal displays with individual pixel or cells 50 microns on a side have been fabricated and displays with cell sizes as small as 25 microns on a side are feasible.